Depleted Uranium(This complements the general fact sheet for uranium.)

What Is Depleted Uranium? Depleted uranium is created as a byproduct during the processing of natural uranium to make it suitable for use as fuel in nuclear power plants or as a component in nuclear weapons. In natural uranium, 99.27% of the mass consists of atoms of uranium-238. About 0.72% of the mass consists of atoms of uranium-235, and a very small amount (0.0055% by mass) is uranium-234. Although uranium-235 is the rarer of the two major uranium isotopes, it is the one that most readily undergoes nuclear fission and is thus the most useful for common nuclear applications. For most of these applications, the proportion of the uranium-235 isotope found in natural uranium must be increased through a process called enrichment.

The specific activity (SA) of a uranium compound depends on its isotopic composition. The SA of natural uranium (containing 0.72% uranium-235) is 6.77×10-7 curies per gram (Ci/g). The SA for other mixtures of uranium-238, uranium-235, and uranium-234 can be estimated using the approach developed by the U.S. Nuclear Regulatory Commission (see 10 CFR 20):

Thus, the SA of depleted uranium is approximately half that of natural uranium. (To express SA in standard international units, multiply the value in Ci/g by 3.7 × 1010 becquerels [Bq]/Ci.)

The uranium enrichment process results in the production of “enriched” uranium (containing >0.72% uranium-235) and “depleted” uranium (containing <0.72% uranium-235). The uranium enrichment process also removes much of the uranium-234 from the depleted uranium. Most depleted uranium in the United States contains between 0.2 and 0.4% uranium-235, with the remainder being uranium-238 and a slight amount of uranium-234. Depleted uranium is less radioactive than natural uranium because some of the uranium-235 and most of the uranium-234 have been removed. Depleted uranium will not undergo a nuclear chain reaction.

How Is It Produced? In the United States, uranium is enriched by the gaseous diffusion process in which the compound uranium hexafluoride (UF6) is heated and converted from a solid to a gas. The UF6 gas is forced through a long series of compressors and converters with porous barriers. Because uranium-235 has a slightly lighter isotopic mass than uranium-238, UF6 molecules made with uranium-235 diffuse through the barriers at a slightly higher rate than do the molecules containing uranium-238 (the uranium-234 molecules diffuse through the barriers at a rate even higher than that of the uranium-235 molecules).

At the end of the process there are two UF6 streams, with one having a higher concentration of uranium-235 than the other. The stream with the higher uranium-235 concentration is referred to as enriched UF6, while the stream that is reduced in its concentration of uranium-235 is referred to as depleted UF6. The depleted UF6 can be converted to other chemical forms, such as depleted uranium oxide or depleted uranium metal. Other methods can be used to enrich uranium, and they too produce depleted uranium as a byproduct. The most common enrichment process used outside of the United States is gas centrifuge enrichment. Laserbased enrichment processes have also been investigated but not commercially developed.

How Is It Used? Although the vast majority of depleted uranium is stored as a byproduct of the enrichment process at the gaseous diffusion plant sites, several current and potential uses exist. Because of its high density, depleted uranium is currently used for radiation shielding. Depleted uranium metal was previously used on large commercial aircraft as counterweights in the wings. Military applications of depleted uranium include use as tank armor, armor-piercing projectiles (antitank weapons), and counterweights in missiles and high-performance aircraft. In a potential future use, depleted uranium could be mixed with highly enriched uranium from retired nuclear weapons to produce nuclear reactor fuel. This process is called blending, and to date only natural or slightly enriched uranium has been considered for this application. Limited amounts of depleted uranium can also be used for the fabrication of mixed uranium and plutonium oxide (MOX) fuel, which is currently being evaluated for use in commercial nuclear power plants.